Various kinds of color filters for display devices have been known in the art, but as typical examples there of, color filters for liquid crystal display devices are taken up for discussion herein.
Liquid crystal display devices are widely used for digital display in electronic calculators and clocks and for analog display in measuring devices, instrumentation, household appliances, and audio equipment. Recently, needs have been increasing for incorporation of color display capability into such display devices, particularly in such applications as peripheral terminal displays in various kinds of equipment, mounted display, telephone display, and TV image display. In attempts to meet said needs, various liquid crystal color display devices have been proposed, some of .hiwch are now being put in practical application. However, none of the color display systems for liquid crystal display devices so far proposed have been successful in meeting the user's needs.
One known type of color filter applicable to liquid crystal display devices having color display capability is such that a transparent substrate has gelatin layers formed in the interior thereof, the gelatin layers being colored by the photolithography technique in different colors, one layer after another.
Any color filter produced by such technique, however, involves the following difficulties. That is, (a) it lacks evenness: since gelatin layers in different colors are laid one over another, level unevenness is likely to be produced; (b) it lacks heat resistance and, therefore, a crack is likely to be caused in the color filter by heating when a transparent conductive film is formed by deposition on the filter. If the deposition step is carried out at a low temperature in order to avoid aforesaid difficulty of cracking, the transparent conductive film is likely to show an increased resistance or allow less transmission of light. And (c) such color filter lacks water resistance, whereby washing the filter with an aqueous solution becomes difficult, it being thus likely that the display performance quality of the liquid crystal is deteriorated by stains. Further, (d) the color filter lacks resistance to weathering, and (e) the manufacturing process for it is complicated. For these reasons, color filters produced by the photolithography method have not satisfactorily met user's needs.
Another construction of a color filter of the type is known which incorporates aluminum oxide layers. For example, one such color filter comprises aluminum oxide anode layers formed in the interior of a transparent substrate, the anode layers being colored (Japanese Published Unexamined Patent Application Nos. 53-99822 and 53-110379), and another comprises porous layers formed by deposition on the interior of a transparent substrate, the porous layers each having a coloring material deposited thereon (Japanese Published Unexamined Patent Application No. 55-166607).
However, these color filters have critical drawbacks in that (a) they have no transparency and that (b) they have poor dye affinity. Therefore, neither of them has been put into practical application.
In view of these facts, the present inventors made extensive studies in an attempt to overcome the difficulties with color filters produced by aforesaid photolithography technique and to provide a color filter which could be put to practical use. As a result, they developed, as their previous invention, a color filter comprising active film layers formed of a colloidal alumina and a colloidal silica, the film layers being dyed (see Japanese Patent Application No. 59-97210, laid open to public inspection on Dec. 14, 1986 under Japanese Published Unexamined Patent Application No. 60-254001). This color filter is produced by repeating, according to the number of colors required, a process which comprises the step of forming on a transparent substrate an active film layer comprised of at least one or more of active alumina or active silica and then mounting on the active layer a metal mask having specified pattern holes, the step of mounting on the metal mask a transfer sheet having an ink layer containing a capable of sublimation dye and/or a hot-melt dye capable of being vaporized, the step of heating under normal pressure to cause the dye in the ink layer to migrate in vapor state to the active film layer so that the film layer is dyed, and the step of removing the transfer sheet and the metal mask, thereby sequentially forming the active film layer patterns corresponding to the pattern holes in the metal mask, finally forming an overcoat layer for closing from above the fine holes in the active film layer.
The color filter produced by this method eliminates aforesaid various drawbacks of the conventional color filters, but involves another set of problems as stated below. That is, since a metal mask is interposed between the transparent substrate and the transfer sheet, it is necessary that the dye be vaporized so as to be allowed to jump a distance corresponding to the thickness of the metal mask, instead of the method of contact migration as in conventional transfer printing. Therefore, the dye requires sufficient considerable energy (heat and time) to overcome the air resistance involved. Accordingly, a considerable amount of deformation due to the expansion of the metal mask is likely to develop, the performance accuracy of the color filter being thereby adversely affected. Another problem is that since the dye used must be readily vaporizable, the range of dye selection is limited, which fact renders it impracticable to use dyes having excellent dyeing power and good heat stability, naturally leading to lowered performance characteristics of the color filter.
One conceivable approach toward solving these problems might be to use a metal mask having less film thickness. However, it was found that this would involve breakage of the metal mask due to its deficiency in strength and would be undesirable from the standpoint of durability.
Then, after their further research into the possibilities of solving aforesaid problems with the prior-art methods, and by improving the foregoing method, which may be called the metal mask method, the present inventors have now arrived at the present invention, overcoming the difficulties with the prior art.
The object of the present invention is to provide a color filter having excellent properties, such as smoothness, heat resistance, water resistance, and light-resistance, and fine patterns with excellent spectral properties for three colors, red (R), green (G), and blue (B), and capable of being manufactured in an easy process, and a method for manufacturing the same.